With the expansion of high-tech industry, the ensuring of metal resources including rare metal has become a big issue. Rare metal is not produced in our country and is totally dependent on imports from foreign countries. The price of rare metal dramatically rises or fluctuates. Therefore, along with the ensuring of overseas resources, the development of alternate materials, and the accumulation of some stockpiles, recycling has been an important issue. Meanwhile, although the concentration is low, a large amount of valuable metals are contained also in the high-tech waste, which is called urban mine, and a project for recovering valuable metals from the waste has been in progress. In the construction of such a recovery/recycling system for valuable metals, the development and establishment of an efficient technique for the removal and separation of valuable metals has been urgently needed.
Generally, for the removal and recovery of metals, methods such as aggregation, coprecipitation, solvent extraction, and solid adsorbing materials have been used. In consideration of the facilities, environmental impact, and also recycling, a method that uses a solid adsorbing material, such as an ion-exchange resin and a chelating resin, is effective. The metal adsorbed by such a solid adsorbing material can be recovered relatively easily by acid cleaning or the like, and the acid-cleaned adsorbing material can be used again for the removal and recovery. These adsorbing materials have been widely used for the removal and recovery of metals. In particular, chelating resins have higher affinity than ion-exchange resins and thus can be regarded as optimal adsorbing materials (Nonpatent Documents 1 to 4). A chelating resin is believed to be capable of removing and recovering a heavy metal element in a solution containing high concentrations of salts, which is difficult in the case of an ion-exchange resin. The ability to form a complex with a metal element differs depending on the functional group structure, and thus chelating resins having various functional groups, such as an iminodiacetic acid group, a low-molecular-weight polyamine group, an aminophosphate group, an isothionium group, a dithiocarbamic acid group, and a glucamine group, are commercially available. Among them, a chelating resin having introduced thereinto an iminodiacetic acid group, which is applicable to the adsorption of a wide range of metals, has been mainly used.
A chelating resin is a particulate adsorbing material like activated carbon and ion-exchange resins and has been used in a wide range of fields including a wastewater treatment and a water purification treatment. A water treatment technique using these particulate adsorbing materials has already been established and is expected to be heavily used also in the future. However, because it has a particulate form, such a particulate adsorbing material has to be packed in a specific can and used. Therefore, it may be difficult to adapt to some conditions of use or some installation environments. That is, in order to meet various demands, not only the adsorption characteristics of an adsorbing material are important, but also the adsorbent needs to be usable in various forms including a particulate form. Methods for producing a chelating resin have already been known (Nonpatent Documents 1 to 4).
According to a typical production method, a chloromethyl group is introduced into a crosslinked, particulate polystyrene by a suitable reaction method, and then a chelating compound such as iminodiacetic acid is introduced by a reaction with a chloromethyl group. In addition, it is also possible to introduce a chelating compound using, as a base material resin, particles of a copolymer crosslinked with a monomer having a glycidyl group such as glycidyl methacrylate. The effectiveness of a long-chain amino carboxylic acid group in such a particulate chelating resin has been disclosed (Patent Document 1 and Patent Document 2). According to the disclosure, the chain length of a chelating functional group is increased, whereby the stability constant of the complex is improved, resulting in the formation of a stable complex. Although an increase in the chain length of a functional group is a technique effective in improving adsorption characteristics, because of the particulate form, the use is limited as mentioned above. In addition, although a chelating polymer can be obtained by introducing an amino carboxylic acid group into a linear polymer having a chloromethyl group or a glycidyl group, the polymer thus obtained is water-soluble and thus is difficult to operate as a solid in a particulate form, etc. Therefore, the operativity is even lower than that of particulate ones.
Incidentally, resin-sintered porous bodies obtained by sintering a powder of a thermoplastic resin such as polyethylene, polypropylene, or polystyrene have been used in various filters and also in air diffusion cylinders, fuel induction cores of gas cigarette lighters, ink induction cores of fiber-tipped pens, ink rollers, foaming devices, etc. By adjusting the particle diameter or particle size distribution of a resin powder, which is the raw material, a resin-sintered porous body can be produced as a porous body having a pore size of five to several hundred micrometers and a porosity of 30 to 50%.
When a particulate adsorbing material is sintered using this technique, adsorbing materials of various forms, such as cylindrical, disk-shaped, needle-shaped, conical, and cup-shaped, can be produced. With respect to a sintered adsorbing material sintered with a thermoplastic resin, activated carbon is disclosed in Patent Document 3, an ion-exchange resin is disclosed in Patent Document 4, and a chelating resin is disclosed in Patent Document 5 and Patent Document 6. These techniques are useful to diversify the form or adsorption characteristics of an adsorbing material. However, it is necessary to perform sintering using a special die set according to the form and the size, and thus a wide variety of dies have to be prepared according to a variety of sizes. Therefore, it is difficult to rapidly deal with various needs. In addition, considering the production method or production facilities, continuous/mass production is difficult, except for flat ones.
In order to solve such a problem, for example, Patent Document 7 discloses a fibrous metal-adsorbing material that can be easily processed into various forms and can meet various demands. For example, Patent Document 7 discloses the introduction of a chelating functional group into a fiber material using a chemical grafting method. Patent Documents 8 and 9 disclose radical formation by radiation exposure and the introduction of a chelating functional group by graft polymerization method. Patent Document 10 discloses a method for injecting a low-molecular-weight chelating agent into general-purpose fibers under high-temperature and high-pressure conditions. These chelating fibers are likely to have sufficient functions and show quick-adsorption characteristics, but have production problems. In a chemical grafting method, the kind of graftable fiber is limited, and also the production process is complicated. A radiation grafting method is advantageous in that it can be applied to various fibers unlike the chemical grafting method. However, for the handling of radiation, the operation is performed in a specific environment, and thus it cannot be regarded as a simple and inexpensive production method. In addition, although a chelating agent injection/impregnation method is also advantageous in that various fibers can be used, according to the disclosed conditions, a supercritical fluid such as carbon dioxide is the most effective, and also the pressurizing conditions include an extremely high pressure of 100 atm (9.8×106 pa) to 250 atm (2.45×107 pa). Therefore, it cannot be necessarily regarded as a simple production method.
Patent Document 11 discloses a method for producing a fibrous metal-adsorbing material using a blend-spinning method. According to this method, a polymer having chelating ability is subjected to wet blend spinning together with viscose, which allows for mass production at low cost using existing facilities. A nonwoven fabric made of this fibrous metal-adsorbing material shows adsorption capacity according to the amount of blend spinning. Therefore, it is possible to produce adsorbents in various forms by secondary processing (Patent Document 12). Further, use of this method makes it possible to produce an adsorbing material in a membrane form, such as cellophane, or in a power form. The polymer to be blend-spun in this method has to be water-soluble and also reproducible uniformly with viscose. In addition, the adsorption characteristics of a fibrous metal-adsorbing material produced by this method depend on the properties of the polymer to be blend-spun. Therefore, in the production of a fibrous metal-adsorbing material having various adsorption characteristics using a wet blend-spinning method, there is a difficulty in that a novel polymer, which satisfies all of such conditions, has to be synthesized each time. Further, the use of a fibrous metal-adsorbing material obtained by this method is also limited. Generally, a treatment solution that is the subject of metal recovery is an acidic solution, and occasionally contains hydrochloric acid, sulfuric acid, nitric acid, or the like within a concentration range. Rayon is decomposed when exposed to strongly acidic conditions. Therefore, the use of a fibrous metal-adsorbing material using rayon as a base material under acidic conditions is significantly limited. In addition, decomposition also occurs due to microorganisms in the environment, etc. Therefore, it cannot stand long-term use or repeated use several times.